1. Field of the Invention
This invention is in the field of forming shaped articles from dispersions and has particular application to the molding of shaped articles from a suspension of solid particles in a liquid medium.
2. Description of the Prior Art
There are materials which are difficult to form into shaped articles by traditional molding processes. These processes typically utilize either heat or solvents, and these can cause deleterious changes in such materials. Examples include the loss of molecular structure which collagen undergoes when heated; the loss of molecular orientation or change in crystallinity or in shape which polyester fibers undergo during molding; th damage to glass fibers which would occur in a molding operation; the loss of fiber shape which would occur if polyacrylonitrile fibers were molded into a mat or nonwoven fabric by prior suspension in a relatively volatile solvent such as dimethyl formamide, etc.
In addition, some materials are infusible and insoluble, which means that the traditional molding processes are simply not applicable.
One well known process for forming articles from infusible, insoluble dispersions is that process used in paper making. In this process, a dispersion of particles of paper pulp is concentrated by a filtration process which occurs using gravity-induced deposition of particles to leave behind a highly concentrated, viscous, gel-like substance which can be subsequently dried to produce sheets of paper. This method, however, relies upon the force of terrestrial gravity, which is unidirectional and which cannot be controlled. Therefore, deposition of the concentrated gel-like substance can only take place on a planar surface, and the process is limited to forming sheets. More complicated shapes cannot be fabricated, except by first forming sheets and subsequently forming the sheets into the more complicated shapes. This invariably results in the presence of seams which cannot be tolerated in many applications, such as the formation of vascular protheses, etc., from biocompatible materials. Seams are also undesirable in the manufacture of containers, e.g., milk cartons, since an additional processing step is required to close them up, often by use of an adhesive or metal staples. Seams also usually constitute areas of inherent mechanical weakness where failure is more likely.
Recently, a new class of biocompatible materials comprising crosslinked reaction products of collagen and a mucopolysaccharide has been disclosed. See U.S. patent applications Ser. Nos. 596,111 abandoned and 596,112, filed July 15, 1976, now U.S. Pat. No. 4,060,081 in the names of Yannas et al. These are infusible, insoluble materials, which are particularly ueful for vascular prostheses, etc. Papermaking processes are unsatisfactory for forming vascular prostheses from these materials, because of the presence of seams which cause blood-clotting when these prostheses are implanted. Other methods for forming articles from collagen-based materials are also not satisfactory.
One widely used method of forming articles from dispersions of collagen involves extrusion, often into a non-solvent such as acetone, alcohol or a salt solution. Many times an extrusion method is used, for example, to form surgical sutures or edible sausage casings from collagen materials. See, for example, U.S. Pat. Nos. 3,123,482; 3,114,372; 3,114,593; and 1,548,504. While extrusion is suitable for certain simple shapes, such as straight tubes, sheets, films, fibers, sutures, etc., it does not lend itself to the fabrication of more complicated shapes such as tubes having one or more bifurcations.
Lyophilization is another shaping technique used to make highly porous articles such as surgical spones or absorbent mats from collagen based materials. See, for example, U.S. Pat. Nos. 3,632,371; 3,471,598; 3,368,911; and 2,610,625. Lyophilization does not offer the degree of control required to form complicated vascular prostheses, however.
Electrodeposition is still another method referred to in the literature which makes use of the fact that collagen molecules carry a net electric charge outside the range of their isoelectric pH and move towards an electrode which can be any desired shape such as a tube. This technique is described in U.S. Pat. No. 3,446,939 as well as in Schmitt, F. O., J. Amer. Leather Chem. Assoc. 46,538 (1951). This technique is not applicable to the forming of crosslinked collagen-mucopolysaccharide materials because complexation and crosslinking of collagen leads to partial or total neutralization of the positive charges carried by the collagen molecule. Additionally, it is also difficult to prepare electrodes having the complicated shapes required for various prostheses formed from the crosslinked collagen-mucopolysaccharide biocompatible materials.